This study investigated the contributions of 5 mechanisms to the effects of preschool participation in the Child-Parent Centers for 1,404 low-income children in the Chicago Longitudinal Study. Based on a matched-group design, preschool participation was associated with significantly higher rates of educational attainment and lower rates of juvenile arrest. LISREL analysis revealed that the primary mediators of effects for both outcomes were attendance in high-quality elementary schools and lower mobility (school support hypothesis), literacy skills in kindergarten and avoidance of grade retention (cognitive advantage hypothesis), and parent involvement in school and avoidance of child maltreatment (family support hypothesis). The model accounted for 58% and 79% of the preschool links with school completion and juvenile arrest, respectively. The maintenance early intervention effects are influenced by many alterable factors.